Electrical supply equipment for discharge tubes



Aug. 15, 1933. L. H. SOUNDY 1,922,984

ELECTRICAL SUPPLY EQUIPMENT FOR DISCHARGE TUBES Filed Feb. 18, 1952 Hf L1 HI MOT/WW5 Patented'Aug. 15, 1933 UNITED STATES ELECTRICAL SUPPLY EQUIPDIENT FOR DISCHARGE TUBES LeonardHenry Soundy, London, England, as-

signor to Uneon Limited, London, England, a

British company Application February 18, 1932, Serial No. 593,889, and in Great Britain May 12, 1931' 6 Claims- (Cl. 176-124 This invention relates to illuminated signs in which use is made of luminous discharge tubes,

and particularly of tubes having a filling of a,

vapour or gas such as neon. The object of the present invention is ,to provide a satisfactory method of energizing such discharge tubes which are to be connected in parallel across a source of supply so that, among other advantages, if one of the tubes becomes disconnected or short-cir-- other words, the starting voltage, applied to a,

tube has to be much larger than the running voltage in order to overcome the initial resistance and to ionize the gas or vapour in the tube, thus rendering it conductive, by a breaking-down process. The use of transformers with open magnetic circuits and consequently large stray fields has been suggested for supplying such tubes because the characteristic of such a transformer ensures that while the open circuit voltage of the secondary winding may be high, as soon as a load current passes to one or more tubes, the available voltage of the secondary winding drops considerably. It has also been sought to increase the elfectiveness of the high starting. voltage by producing a powerful starting surge, and in particular it has been suggested to employ for this purpose a condenser in the primary circuit or the secondary circuit of such a transformer, the con r self-regulating circuit arrangements for automatically controlling the voltage availabe from a circuit in which the conditions approach electrical resonance.

Thus, according to the present invention, a coil and condenser are included in a circuit to which a low tension alternating current-is supplied, and their inductance and capacity respectively are such in relation to the frequency of the alternating current, or to a harmonic of that frequency, that a potential difference due to electrical resonance effects is available for starting and operating one or more discharge tubes, while in series with that tube or each of the tubes, there" is a suitable impedance of such a character that when the tube and the impedance are brought into circuit, the said potential difference remains substantially constant at least or even increases. The voltage attained in this way may be much. higher than that of the source of supply as an amplification factor as high as 30 may be easily obtained in the case of a supply having a frequency of 50. cycles per second. The actual inductance in the low tension circuit may consist only of a portion of the winding of a'coil with an iron core, and the tube or tubes to be illuminated are then connected across the outer terminals of that coil.

In general the impedance in series with the tube or with each tube, will include both induc tance and capacity, and it is believed that the maintenance of the potential difference or the rise in potential diiference may be due to the fact that the switching in of a tube and its series impedance causes the conditions in the circuit to approach more nearly to electrical resonance.

In order to deal with the conditions on starting the tubes, the voltage available is much higher than is necessary for normally supplyingthe tubes, but the inductance and capacity in series with each tube have such magnitudes that after a tube is started, the potentialdiiference across its terminals is only that required for running conditions. In other words, at the moment of switching on or introducing an additional discharge tube, the almost infinite resistance of the tube is the controlling factor in the branch in-. cluding the tube so that the full voltage produced by the resonant circuit is available for breaking down the resistance of the tube. As soon, however, as the tube strikes and its resistance drops very considerably, the voltage conditions in the branch circuit are controlled by the impedance consisting of the condenser and inductance. V

In explanation it maybe stated. that if the inductance in the circuit connected to the alternating current supply is less'in relation to the capacity in that circuit than is required for resonance conditions, it may be demonstrated mathematically that the effect of adding shunt circuits each with a luminous tube in series with an inductance and condenseras described above,

is to make the system more nearly approach the a branch, an effective increase is caused in the inductance in the circuit connectedv to the alternating current supply. which increase depends upon the value of the capacity of that condenserdrawing, which is a diagram of connections of thenovel circuit arrangement.

In this diagram, alternating current mains,

which may be considered as 240 volts mains givinga supply at a frequency of 50 cycles per second are shown at Ml and M2. The main inductance is an iron-cored choke coil W with a tapping point connected to one of the mains M2. 'The portion of this coil Lp which is in the mains circuit in the particular example under consideratlon,is wound with 1,500 turns of 21 gauge wire, while, the portion of this coil Ls on the other side of the tapping point is wound with 3,000 turns of 30 gauge wire, the core having a cross-sectional area fof about 1.5 square inches, so that the'approximate in-v ductances of the two parts when they aredisconnected are 5 henries and 20 henries respectively. The respective resistances of the portions Lp and Ls of the coil are about 16 ohms and 250 ohms. The condenser C in series with the mains and the portion Lp of the coil W has a capacity of 3 mid.

Bus-bars B1, B2 are run from the terminals of the coil W, and the tubes T1, T2, T3 are connected across these bus-bars in parallel branches; three tubes are shown merely as an example, and naturally further tubes can be added as desired. The inductances L1, L2, L3 in series with'the several tubes have a value of about 45 henries each with a magnetising current of 18 milliampres, the direct current resistance of each coil being 775 ohms. The condensers C1. C2, C3 in each branch have a capacity each of .01 mfd. In order to afford some adjustment of the voltage avail able, a variable resistance R of afew ohms is connected in the low tension circuit from the mains. It was found that with no load'across the busbars, the current flowing in the low tension cir-'.

cuit was 30 milliamperes. I

Eaclrglement comprising a discharge tube such as T1 with its series condenser Cl and choke coil L1, may be mounted in a single box or casing so that the only connections made to the outside of each casing are those from the insulated busbars B1, B2 and the continuation of the busbars which pass on to the next unit of the system. Upon switching on the current, the approximate series resonance existing in the low tension supply circuit by reason of the relationship of the capacitative reactance of the condenser C to the inductive reactance of the coil W, causes an amplified potential difference to be established across the portion L and thus an amplified potential difference is available at spaced points of the coil W, e. g. on the bus-bars B1, B2. Each tube receives an initial striking voltage at least equal to the potential difference across the choke coil W, but directly a tube becomes illuminated and current commences to flow through it, the

' to 1,356 volts.

. tubes current is controlled by the condenser C1, C2 or C3 in its circuit, which becomes the controlling impedance in each branch. The condensers C1, C2, or C3 in the branch circuit, however, also operate, so to speak, by modifying the effective reactance in the low tension supply circuit corresponding to a change in the inductive reactance of the coil, and thus operate to bring the capacitative and inductive reactance of the low tension supply circuit more nearly into series resonance, so that each branch circuit may be considered as having an impedance cooperative with the reactance in the supply circuit for bringing the supply circuit nearer to series resonance when the device is in normal operation, or at least cooperating therewith for preventing departure of this supply circuit away from series resonance.

In order to explain the action of self-regulation of the circuit as additional tubes are added, it may be mentioned that in a particular example, the voltage across the main condenser C was approximately 660 volts and across the inductance Lp in the, low tension circuit about 524 volts. The open circuit voltage across the bus-bars B; B2 was 1,340 volts. With a single tube T1 across'the bus-bars in series with its inductances L1 and capacity C1 of the magnitudes mentioned above, the voltage across the terminals of the coil W increased to 1,346 volts; when two equal tubes 3 feet long were connected with their branch circuits in parallel, the voltage increased The current through the first tube was 12 milliamperes in both cases, and the current through the second tube was 13.5 milliamperes. When two unequal tubes were connected in parallel branches, one tube being 3 feet long and the other 1 foot six inches long, the voltage across the coil W was 1,360 volts, the current in the first tube 12 milliamperes and in the second tube 13.5 milliamperes. In a further test, tubes T1, T2 each 3 feet long were connected in parallel branches and three similar additional connected in three further parallel branches. .The current in the tube Tl was 12.3 milliamperes and in the tube T2 13.6 milliamperes, while the voltage across the terminals of the coil W now increased to 1,380 volts.

It is clear that with the dimensions set out above,- the feed system to the tubes acts as a compounded system with a slightly rising voltage characteristic curve, while the current through any one tube does not decrease when further tubes are added in parallel. The amount of therise in voltage when the number of tubes is increased is entirely under control as it depends entirely upon the design of the choke COilS L1, L2, L3.

The invention is not, of course, limited to the precise system shown in the diagram. For example, the ratio between the parts of the coil W would be modified to suit the voltage of the mains and the voltage required for the tubes, and even the whole of the coil W might be included in the low tension circuit connected to the mains.

I claim:-

1. An electrical supply equipment for luminous tube and like devices which require a high potential for starting, comprising a supply circuit connected to a low tension source and including a coil and a condenser, and circuit connections from spaced points of said coil to a said device and including an impedance, said coil and condenser being near series resonance at a frequency of the alternating current flowing in said supply circuit whereby an amplified potential difference is set up between said points of said coil for starting said device, and said impedance having a reactance cooperative with the reactance of said supply circuit for preventing departure of said supply circuit away from series resonance when said device is in normal operation.

2. An electrical supply equipment for luminous tube and like devices which require a high potential for starting and a lesser potential for normal operation, comprising a supply circuit connected to a low tension source of potential insufficient for starting a device and including a coil and a condenser, and circuit connections from spaced points of said coil to a said device and including means providing an impedance, said coil and condenser being near series resonance at a frequency of the alternating current flowing in said supply circuit so that an amplified potential difference is set up between said points of said coil for starting said device, and said impedance having a reactance cooperative with the reactance of said supply circuit for bringing said supply circuit near to series resonance when said device is in normal operation, said impedance functioning during such normal operation to prevent overrunning of the device.

3. An electrical supply equipment for luminous tube and like devices which require a high potential for starting and a lesser potential for normal operation, comprising a supply circuit connected to a low tension source and including a coil and a condenser, and circuit connections from spaced points of said coil to a plurality of said devices, each connected in an independent parallel branch circuit, said coil and condenser being near series resonance at a frequency of the alternating current flowing in said supply circuit whereby an amplified potential difference is set up between said points of said coil for starting any of said devices, each said branch circuit having a reactance cooperative with the reactance of said supply circuit for preventing departure of said supply circuit away from series resonance when said corresponding device is in normal operation.

4. An electrical supply equipment for luminous tube and like devices which require a high potenial for starting and a lesser potential for normal operation, comprising a supply circuit connected to a low tension source and including a coil and a condenser, and circuit connections from spaced points of said coil to a said device and including a second condenser, said coil and condenser being near series resonance at a frequency of the alternating current flowing in said supply circuit, the inductance of said coil in the supply circuit being less in relation to the capacity in said supply circuit when said connections are open-circuited than is required for resonance, said second condenser having a reactance cooperative with the reactance of said supply circuit for preventing departure of said supply circuit away from series resonance when said device is in normal operation.-

5. A supply equipment as in claim 4, in which said second condenser and a choke coil are connected in series circuit with said device by said circuit connections, said choke coil operating to prevent overrunning of said device under the amplifled potential difference.

6. An electrical supplyequipment for luminous tube and like devices which present a high resistance at starting and a lesser resistance during normal operation, comprising a supply circuit connected to a low tension source of potential insuflicient for starting a device and including means for providing capacitative and inductive reactances which are near series resonance at a frequency of the alternating current flowing in said supply circuit, and circuit connections from said coil to a plurality of said devices and including individual branch circuits for said devices, each said branch circuit including means for providing an impedance cooperating with the reactance of said supply circuit for bringing the supply circuit near to series resonance as each said device comes into normal operation and for limiting the potential difierence across the terminals of the associated device during normal operation.

LEONARD HENRY SOUNDY. 

